This invention relates to an in-line two cylinder engine of the kind comprising a crankshaft having crankpins to which the ends of connecting rods connected to the pistons are articulated, the crank pins being set at 360.degree., the engine having perpendicular to the crankshaft axis a central plane in which lies the resultant of the reciprocating inertia forces to be balanced, the engine also having balancing means in the form of a first counterweight rotatable around a rotational axis parallel to the crankshaft in the opposite direction to, but at the same speed as, the crankshaft, the crankshaft being so set that its centre of gravity is remote from the crank pins when the pistons are at top dead centre, the centre of gravity of the counterweight being disposed in the central plane of the engine, the first counterweight being of a size and construction such as to develop a centrifugal force whose amplitude is half the maximum amplitude of the reciprocating rectilinear inertia forces of the engine, the rotational axis of the first counterweight being disposed outside the plane defined by the axes of the cylinders.
It is an object of the invention to improve the practical performance of such engines, inter alia to improve their balancing, to simplify the balancing means and to improve the accuracy of the various settings or adjustments determining the effectiveness of the balancing.